1. Field of the Invention
The present invention relates to an X-ray diagnostic apparatus for obtaining moving images.
2. Description of the Related Art
An X-ray diagnostic apparatus of this type can examine motions inside the body of a patient by fluoroscopy in real time; that is, an X-ray diagnostic apparatus images X-rays transmitted through a patient repeatedly at a period of 30 frames per second by using a TV camera via an image intensifier or a fluorescent screen, while radiating pulse X-rays repeatedly at a period of 30 times per second. This fluoroscopic technique is essential in diagnoses using a catheter.
In this catheter diagnosis, however, not only patients but also doctors and nurses cannot avoid exposure. To reduce the exposure dose, two methods are possible. The first method is to decrease the energy of pulse X-rays. Unfortunately, the first method has the inconvenience that the S/N ratio decreases with decreasing energy, and this results in degradation of the image quality. The second method is to decrease the number of irradiation times per unit time (one second) without lowering the energy of pulse X-rays. The second method, however, has the disadvantage of being unable to smoothly observe motions inside the body of a patient, and also has the following problem. Generally, TV cameras are so designed as to have a frame rate of 30 frames/sec as a basic specification. If the number of irradiation times is reduced to be smaller than 30 per second while the frame rate is kept unchanged at 30 frames/sec, photoelectric conversion periods in which no X-rays are radiated take place periodically. Output signals obtained from a TV camera during this period have no gray-scale information. Therefore, white images periodically mix in moving images to make observations very difficult. Note that orders of the frame rate other than 30 frames/sec are unpreferred since the cost of a TV camera is extremely increased.